A deterioration determination method of a lithium secondary battery is disclosed in Patent Document 1. In the deterioration determination method, an estimated capacity of the lithium ion battery is calculated from a voltage change during charge, and when the estimated capacity is smaller than a limit capacity which is set in advance, the lithium secondary battery is determined as being deteriorated.
In addition, a battery monitor device for detecting a battery state is disclosed in Patent Document 2. The battery monitor device has a function of setting a balancing switch to be in an off state during a measurement period of a cell voltage.